100 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



of somewhat different size groups and from differ- 

 ent habitats, may influence the average volume of 

 stomach contents. 



In general, there was an increase in the mean 

 food volume with an increase in the fork length 

 of the fish (table 5, fig. 4). The average stomach 

 content per unit of body weight (cc./lb.) was 

 found to decrease, however, as the weight of the 

 fish increased (fig. 5). Except for possible aber- 

 rancies at the extreme ends of the polygons and 

 in the 1,250-mm., approximately 84-lb., group, 

 which may be due to quirks of sampling, there 

 appears to be a rectilinear relation between the 

 volume of stomach contents and size of the fish. 



The great variation in volume of stomach con- 

 tents is illustrated in table 5. For the various 

 size groups represented, the standard deviation 

 (s) is roughly proportional to the mean volume 

 (x), indicating a great increase in variance, or 



variation about the mean, of volume of stomach 

 contents with increase in fork length. This vari- 

 ation is related to the fact that large yellowfin eat 

 organisms of greater dimension than those con- 

 sumed by small yellowfin, but that both groups 

 feed on the same minute organisms, such as crab 

 and stomatopod larvae. Therefore, the larger the 

 yellowfin the greater the size range of the indi- 

 vidual food elements and the greater the range in 

 volume of stomach contents that may be expected. 



Variation in food of yellowfin related to locality, habitat, 

 and body size 



As indicated in tables 1 and 2 and figure 1, 

 yellowfin were captured by three methods, came 

 from different depths and localities, and covered 

 a wide size range. It was difficult, therefore, to 

 find within this heterogeneous lot of fish, groups 

 that might be compared to show differences re- 

 sulting from the action of a single variable. 



NUMBER OF SPECIMENS 



Z 

 O 



i 



li. 

 o 



z 

 tu 



Q 



z 



< 



Z 

 O 



S 



> 



w 



Q 

 O 



§ 



Z 



320 

 300 

 260 ' 

 260  

 240 - 

 220 

 200 



ISO 



160 



140 



120 



100 



80 



60 



40 



20 



43 



135 



239 



132 



83 



128 



75 



- r - 



120 



— I — 



29 



COEFFICIENT OF VARIATION (C.0S%) 



STANDARD DEVIATION (S) 



MEAN (5) VOLUME PER STOMACH 

 PERCENT EMPTY STOMACHS 



50 



I 

 o 

 < 



5 

 O 



DC 

 Q. 



| 



3 



i- 



o 

 z 

 < 



E 



5 



750 



950 



1050. 



1150 



1250 



1350 



1450 1550 



1650 



FORK LENGTH IN MM. 

 26 37 50 65 84 



BODY WEIGHT IN POUNDS 



131 



Figure 4. — Relation of percent of empty stomachs, mean volume of food per stomach, standard deviation, and coeffi- 

 cient of variation for volumes of stomach contents to body size of 1,097 yellowfin tuna. 



